This invention relates generally to portable unit construction modular bridging in which components are joined together on site to form a bridge of the required length and strength within the overall limitations of the system.
Examples of prefabricated unit construction modular bridging systems may be found in U.K. Patent No. 553,374 and U.S. Pat. Nos. 3,062,340 and 4,706,436. The Bailey Bridging system shown in U.K. Patent No. 553,374, used extensively throughout World War II, was one of the first and possibly the best known example. In the Bailey system, main load-bearing side girders are built up from rectangular panels (10 feet long and 4 feet 9 inches high center to center of pin-hole connections in the case of Bailey, but possibly differing in other systems) pinned or bolted end-to-end at their top and bottom chords to form a truss of the required length. Panel trusses can be placed side-by-side to form multi-truss single story side girders, and can be bolted together one on top of the other where multi-truss double story construction is required in the side girders.
It follows that any span can be built in multiples of the panel length (10 feet in the case of Bailey Bridging) and the load carrying capacity can be varied, as necessary, by using one or more trusses side-by-side in each side girder of the bridge (multi-truss single story construction) and by bolting panels one on top of the other to form side girders of twice the height and much greater moment of inertia and section modulus (multi-truss double store construction). It should be noted that in order to connect these panels together end-to-end and one on top of the other to form double story trusses, it is essential that each panel be exactly rectangular and precisely identical to the next.
One inherent fault of such 2-pinned rectangular panel bridge trusses is that when adjacent panels are pinned or bolted together at the top and bottom chord connections to form the main supporting side girders of the bridge, sag of the side girders occurs. This pin-hole sag is a function of the actual and necessary tolerance between the pin or bolt diameter and the diameter of the pin (or bolt) hole in the connecting blocks of the adjacent panels, the number of panels (and therefore panel connections) in the span of the side girder trusses, and the distance between the top and bottom chord pin-hole connections. In long span single story bridges this pin-hole sag (when added to normal elastic deflection of the structure under dead and live load) is often aesthetically unacceptable, and this may even be true of double story bridges of very long span even though the pin-hole sag is virtually half that of a single story bridge of the same span and the elastic deflection under load is also reduced due to the greater depth (and increased moment of inertia) of the side girders.
Where such existing 2-pinned rectangular panel bridging systems are required to take heavY loads over long spans, it is usually necessary to increase the bending moment capacity and reduce the deflection of each side girder by bolting the bottom chord of a second row of panels on top of the top chord of the lower row of panels in each truss, as previously described. This form of construction of existing modular bridging has the disadvantage of placing a considerable area of steel (the top chord of the bottom panels and the bottom chord of the top panels) at or near the neutral axis of each truss (which within the context of this patent application can be defined as that horizontal line running through the centroid to the [vertical] cross-section of each truss), and in the majority of cases this superfluous steel merely adds to the dead weight of the bridge and correspondingly detracts from its live load capacity.
Longitudinal forces (due to traction etc.) on the roadway of any through-type bridge must be transmitted from the deck units through the supporting cross-beams (transoms) and the load-bearing side girders to the abutments. In many existing designs of modular truss bridges, the attachment of cross-members (transoms) to the individual panel diagonal/vertical members and bottom chord members causes some rotational forces and residual local bending stresses in one or more of these members. Such effects are detrimental in that they detract from the ability of the diagonal/vertical members and bottom chord members of side panels to withstand the direct compression and tension due to the normal vertical dead and live loads on the bridge.